Electrolytic process of preparing cyclohexadiene dicarboxylic acids



Patented Aug. 2, 1949 ELECTROLYTIGV PROCESS PREPARING CYCLOHEXADIENE DICARBOXYLIC ACIDS Paul c. Condit, ii'erkjeiey, Calif., assigner focaliforna Research Corporation, San Francisco, Calif., a corporation of Delaware Application December 1, 1945, Serial No. 632,171

3 claims.v (c1. :ZM-45) This invention relates to a process of preparing cyclohexadiene dicarboxylic acids by electro'- lytic reduction of a phthalic acid and, more' particularly, to the production of A3,5-cy`c1ohexadiene trans-dicarboxylic acid-1,2 by mercurial cathodic reduction of orthophthalic acid.

The following chemical equation exemplifies the reaction for producing cyclohexadiene dicarboxylic acids from a phthalic acid according to this invention:

It has been found that production of cyclohexadiene dicarboxylic acids by such an electrolytic partial reduction of phthalic acid involves several problems; for example, when the reduction is. elTected at a lead cathode in an aqueous sulfuric acid catholyte, the process results in the formation of a sticky, brown tar with concurrent discoloration of the reduced product. Likewise, the reduction rate becomes relatively slow in the range of 70% to 90% conversion, and it may even become almost infinitely slow (or, not goat all but actuallylead to some destruction of desired product) when the amount of phthalic acid converted to -cyclohexadiene acid reaches about 90% or more. This lo-w rate of reduction in turn yields a very low current eiilciency which inordinately increases the cost of the product, even in the '70% to 90% conversion zoneand renders more than 90% conversion almost, ii not actually, prohibitive. Again, if the process is operated toeffect a lower conversion, e. g., 50% or less, and obtain higher current efficiency, troublesome recovery problems are encountered in the separation of the reduced product from relatively large quantities of unreduced product remaining in the catholyte solution by reason of the low conversion.

This invention is based upon the discovery that tar formation and discoloration of the product may be minimized or avoided in the electrolytic reduction of phthalic acid. The invention also involves the discovery that substantially increased rates of reduction, enhanced current efliciency, and superior selective conversion to the desired product can be obtained in the manufacture of cyclohexadiene dicarboxylic acids by partially reducing the benzene ring of a phthalic acid in the presence of metallic mercury, e. g at a mercury cathode, in a suitable aqueous acid catholyte.

.of such character that it will not dissolve the electrodes under the conditions existing during electrolysis or convert the reaction product of the reduction to an undesired derivative;

Reference is made to the accompanying drawing which shows in more or less diagrammatic form one type of electrolytic cell suitable for carrying out the process of this invention. The apparatus comprises a container I of material capable of resisting the action of the electrolytes, such as glass, the anode 2, and the mercurial cathode 3. yThe anode 2 preferably is made of rolled sheet chemical lead and may be in the form of a cylinder or spiral provided with openings at its lower end to permit circulation of anolyte therethrough. The mercury of the cathode desirably should be a `product of high purity, preferably doubly distilled metallic mercury.

Inspection of the drawing willl reveal a diaphragin 4 in the form of a porous cup, preferably of unglaed porcelain, surrounding the anode 2 and serving to divide the cell into an anode coinpartrnent and a cathode compartment. This diaphragm, together' with its enclosed anode, is supported above but in Vproximity to mercury cathode 3 by an insulating glass ring tripod 5. A suitable source of direct current (not shown) is provided and is connected with anode 2 by means of positive electrical lead B attached to an upwardly eiiitendingV portion of the anode. A negative direct current lead 1 passes through glass tube 8 to' a tungsten wire 9 which is in electrical contact with the mercury cathode and which is sealed in a glass tubel 8 to exclude cell fluids. A stirrer I0 and thermometer Il are here shown, and suitable means for adjusting and controlling the ow of electric current through the cell may be provided in the usual manner. The entire electrolytic cell, as here shown, is immersed in a water bath l2 for the control of cell temperature and particularly catholyte temperature. Desirably, thermostatically controlled heaters (not shown) are provided in the water bath to maintain cell temperature at preferred levels. A chemical lead sheet I4 covers the cathode compartment and, in

some instances, cooling coils may be immersed in the anolyte to furnish auxiliary temperature control and avoid overheating in the anode compartment.

The anolyte and catholyte comprise a dilute aqueous Inineral acid solution, preferably sulfuric acid in water, and the anolyte level l3may be maintained slightly above the level I of the catholyte with the advantage that electrolyte migration through the porous diaphragm will be main tained in a direction from the anode compartment to the cathode compartment, whereby loss of product to anolyte is reduced.

Phthalic acid to be reduced is dispersed in the catholyte, as by dissolving xphthalic anhydride therein. The phthalic anhydride is hydrolyzed to phthalic acid during such dissolution. The term "dispersed or dispersion is herein used in its generic sense to include suspensions, emulsions or true solutions.

'I'h'e reduction should be carried out under conditions whereby gassing and loss of hydrogen are minimized, if the desired current eiciency is to be obtained. Optimum current density inthe cell illustrated is a function of the concentration of phthalic acid in the catholyte; if the concentration is low, high current densities are inefficient because under such conditions free hydrogen gas is formed and the resulting loss decreases the emciency of the process. At higher temperatures the solubility of phthalic acid in the catholyte is increased and greater current densities may be employed under such conditions with high cell eiciency. As the reduction proceeds the concentration of phthalic acid decreases and that of the conversion product, cyclohexadiene dicarboxylic acid, increases thus tending to promote evolution of hydrogen gas. Despite the fact that the concentration of the diene dicarboxylic acid increases during the process, it has been found that the benzene ring of the phthalic acid canv be, selectively reduced and that selectivity can be maintained even above 80 to 90% conversion in the presence of a mercury cathode.

Specific process conditions mayvary with different cell structures, with ratio of volume of catholyte to surface area of cathode, as well as with other variables such as temperature, concentration of sulfuric acid in the electrolyte, and

concentration of phthalic acid in the catholyte,

as above discussed. However, to illustrate suitable conditions for operating the cell shown in the drawing, the following data are given:

Preferred operating temperatures for the catholyte are from about 80 C. to about 90 C., although temperatures as low as about 60 C'. and as high as about 100 C. may be utilized. The concentration of sulfuric acid in the catholyte may vary from about 3% to about 20% concentrated sulfuric acid in water, and the anolyte is preferably approximately the same concentration. From about 2% to about 10%, preferably about 4%, by weight of phthalic anhydride is dissolved or dispersed in the aqueous sulfuric acid catholyte solution. Current densities may vary widely from about 2 amperes per square decimeter to about 40 amperes per square decimeter, it having been found that about amperes per square decimeter is desirable where the catholyte contains approximately 5% sulfuric acid and about 4.5% phthalic acid.

The following data will serve to illustrate the mode of operation and the advantages of a process embodying the principles of this invention. AA cell constructed in accordance with Figure 1 of the drawing utilizing 120 cc. of anolyte, 750 cc. of catholyte, and a mercury surface cathode area of about 200 sq. cm. was provided. 30 g. of phthalic anhydride were dissolved in the 750 cc. of catholyte which consisted of 5% sulfuric acid in water. Temperature Was maintained at to 87.78 C. in the catholyte and 10 amperes direct current were passed through the cell (current density about 5 amperes per square decimeter). In order to follow the progress of the reaction, samples were pipetted from the cathode compartment at definite intervals and analyzed for A3,5 cyclohexadiene trans-dicarboxylic acid-1,2 by titration with bromine. After completion of a run, the catholyte was removed from the cell and chilled to a temperature just above its freezing point. The major portion of the A3,5cyclohexa diene trans-dicarboxylic acid-1,2 crystallized out, was ltered off, and dried. This product was weighed and analyzed for its content of the cyclohexadiene acid and the filtered catholyte was analyzed for its content of dissolved A35-cyclohexadiene trans-dicarboxylic acid-1,2. Material balances were then obtained from these data.

The following comparative data on two similar runs, one with a lead cathode and the other with a mercury cathode, were made in the above manner. Despite the fact that the lead cathode had about twice the surface area of the mercury cathode, which put the mercury at a disadvantage in otherwise duplicate runs, the latter gave faster reduction, and higher conversion as shown by the following:

Table I 1 Per Cent Conversion to A3,5acid Time, at hours lead mercury cathode cathode 1 Both runs at 185 to 190 F., l0 amperes, 40 g. phthalic anhydride per liter 5% H2804 catholyte, 750 cc. catholyte,

The foregoing data were plotted, a curve drawn through the points for each cathode, and the time required for a given per cent conversion to A3,5cyc1ohexadiene trans-dicarboxylic acid-1,2 read from these curves. These last data, showing the per cent time saved for a, given per cent conversion, are given in Table II.

These data illustrate the greatly enhanced effectiveness of mercury cathode reduction of phthalic acid, especially in the range where relatively high conversions to A3,5cyclohexadiene trans-dicarboxylic acid-1,2 are to be obtained with a corresponding predominating proportion of this compound as compared with that of the phthalic acid in the catholyte mixture. It should be noted that the data of Table II are qualitatively correct but are not to be taken as exact experimental determinations since they represent values read from curves plotted from data. But the curves and these data d-o denitely show that, at 85% conversion, for example, about 33% saving in time of reaction resulted with a metallic mercury cathode as compared with a lead cathode. This saving in time corresponds directly to current eiiciency since current through the cell was held substantially constant at amperes. Saving in time thus means a similar saving in cost of electrical current. This saving in time also correspondingly increases cell capacity.

A feature not revealed by the foregoing data involves tar formation. Runs made with a lead cathode, yielded appreciable quantities of a sticky, brown tar, causing discoloration of the conversion product and indicating undesirable side reactions when reduction is effected in the presence of a cyclohexadiene dicarboxylic acid 'l at a lead cathode. But in all runs in substantially the same cell with the substitution of a mercury cathode, no tar formation has been noticed and the products have been lighter in color. No loss of selectivity in reduction was observed even though a relatively high proportion of the highly unsaturated conversion product, a, diene dicarboxylic acid, was present. Further, it has been discovered that the mercury cathode is more resistant to cell poisoning, as reflected by a lower poisoning rate, than is a lead cathode.

The process of this invention is applicable to the selective partial reduction of the benzene ring of other phthalic acids. For example, terephthalic acid may be reduced to A2,5-cyclohexadiene dicarboxylic acid-1,4 according to the chemical equation:

COOH COOH Electrolytic Reduction C O OH Terephthalic Acid Iclairn:

1. A process of electrolytically reducing phthalic acid to produce a cyclohcxadiene dicarboxylic acid which comprises forming a catholyte consisting essentially of about 2% to about 10% by weight of said phthalic acid and dilute aqueous sulphuric acid o1 about 3% to about 20% concentration, contacting said catholyte with a cathode consisting essentially of a body of liquid mercury in an electrolytic cell, maintaining the temperature of said catholyte between about 60 and about C., and passing an electric current of a density of from about 2 to about 40 amperes per square decimeter of cathode area through said cell with said body of liquid mercury as the cathode.

2. A process of electrolytically reducing orthophthalic acid to produce A3,5cyclohexadiene dicarboxylic acid-1,2 which comprises forming a catholyte consisting essihtially of about 2% to about 10% by weight of said phthalic acid and dilute aqueous sulphuric acid oi' about 3% to about 20% concentration, contacting said cathu olyte with a cathode consisting essentially of a body of liquid mercury in an electrolytic cell, maintaining the temperature of said catholyte between about 60 and about 100 C., and passing an electric current of a density of from about 2 to about 40 amperes per square decimeter of cathode area through said cell with said body of liquid mercury as the cathode.

3. A process of electrolytically reducing terephthalic acid to produce A2,5cyclohexadiene dicarboxylic acid-1,4 which comprises forming a catholyte consisting essentially of about 2% to about 10% by Weight of said phthalic acid and dilute aqueous sulfuric acid of about 3% to about 20% concentration, contacting said catholyte with a cathode consisting essentially of a body of liquid mercury in an electrolytic cell, maintaining the temperature of said catholyte between about 6G and about 100 C. and passing an electric current of a density of from about 2 to about 40 a'mperes per square decimeter of cathode area through said cell with said body of liquid mercury as the cathode.

PAUL C. CONDIT.

REFERENCES CITED The following reference-s are of record in the file of this patent:

UNITED STATES PATENTS Name Date Creighton Dec. 28,1926

Mettler, Berichte der Deutsche Chemische Gesellschaft, 39, 2933-2942 (1906).

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